Solar and open sun drying of untreated and pretreated banana stalk chips biomass: a sustainable processing of biomass using renewable solar energy

The stalk of banana is an abundant biomass that can be sustainably converted into bioenergy, biofuels, biosorbents, animal feeds and fibers. The moisture content of freshly harvested banana stalk is high, so drying is essential before its storage and prior to some of the conversion processes. Hence, solar and open sun drying characteristics of banana stalk chips were investigated. Untreated (5, 10 and 15 mm thick) and pretreated (hot water, salt water and sulphite) banana stalk chips were dehydrated in a solar dryer and directly in sunlight. The data were fitted to twelve drying models. The moisture diffusivities were also evaluated. Banana stalk dried mainly in the falling-rate phase and the rate of drying increased with decreasing chip thickness. However, the chips dried slower in direct sunlight compared to the solar dryer. The pretreatments significantly (P < 0.05) enhanced the rate of dehydration of the chips in the solar dryer. Diffusivities for the solar and direct sunlight drying of banana stalk chips were 1.28 × 10–9–5.32 × 10–9 m2 s−1 and 1.08 × 10–9–3.65 × 10–9 m2 s−1, respectively. The Weibull model aptly described the solar drying of the chips while the Midilli-Kucuk and Weibull models most properly depicted the open sun dehydration of untreated and pretreated chips, respectively. Solar energy may be more efficiently utilized for drying banana stalk chips biomass by using a solar dryer as well as a very thin layer of chips and by pretreating the biomass prior to the drying operation. The effective use of solar energy for drying banana stalk biomass can reduce its processing cost and enhance the sustainability of biomass utilization. Banana stalk chips biomass were dried in a natural convection direct solar dryer and open sun. Thinner biomass chips dried faster than thicker chips in both the solar dryer and direct sunlight. Hot water, salt water and sulphite pretreatments significantly enhanced the drying rate of the biomass in the solar dryer. Banana stalk chips biomass were dried in a natural convection direct solar dryer and open sun. Thinner biomass chips dried faster than thicker chips in both the solar dryer and direct sunlight. Hot water, salt water and sulphite pretreatments significantly enhanced the drying rate of the biomass in the solar dryer.

The efficiencies of pyrolysis, gasification and direct combustion processes of converting biomass into biofuels and bioenergy are constrained by excessive moisture in the biomass, thus drying is needed to decrease moisture in the biomass prior to carrying out these processes [29][30][31].Drying is also required before the torrefaction of banana stalk [9].The conversion of banana stalk into adsorbents and animal feeds also requires drying pretreatment [18][19][20]27].In addition, freshly harvested banana stalk should be dried to preserve it from lost to deterioration and decay by microbes.
Drying is a technique utilized to decrease moisture present in agro-products thereby lowering the activities of microbes and enzymes leading to the enhancement of material shelf-like, in addition to the lessening of the cost of packaging and transporting products [32].
Drying in direct sunlight or in open sun is the conventional or traditional technique through which agro-products are dried using sun radiation.Solar radiation is abundant and freely available, so this method is relatively inexpensive.However, drying in direct sunlight depends on weather condition, dehydration time is long, and products being dehydrated are left open to dust, rodents, insect, rainfall, etc. [33][34][35].
Solar drying, like drying in direct sunlight, employs solar energy to remove moisture from a material; but, in contrast to open sun drying method, it uses a solar dryer.This is an equipment that possesses a heating compartment where the wet material is placed, safe away from rodents, insects, rainfall, dust, etc., and heated by the radiation from the sun [33][34][35][36][37].A solar dryer can dry a material in a shorter period compared to drying in direct sunlight because the temperature inside this dryer can be much higher than that of the external environment.
Solar dryers of different types have been employed in the drying of agricultural produce; they include natural convection (passive), forced convection (active) and mixed convection solar dryers (based on the mechanism of hot air contact or mode of air circulation), direct, indirect and mixed heat reception mode solar dryers (based on the reception of the solar radiation) as well as the hybrid solar dryers [38][39][40][41][42][43][44].
Since solar and direct sunlight drying techniques use renewable energy from the sun, these techniques are sustainable especially in regions of high sun radiation (e.g.Nigeria and other African countries) compared to mechanical dryers that use electricity generated from highly polluting and non-renewable fossil fuels [38,39,45].More importantly, the use of abundant and free solar energy in the drying step of biomass processing can reduce the cost and enhance the sustainability of biomass utilization.Hence, it is important to investigate the solar drying of agro-materials in these high solar radiation regions in order to optimize the solar drying processes.
Thin layer drying is a dehydration approach that entails the dehydration of materials as one layer of slices or particles, such that the thickness or depth of the material being dried is small; the material thus possesses a thin structure for faster moisture diffusion out of the material [46,47].The drying rate of a material is significantly influenced by its thickness, so thin layer drying of agro-materials is a very important study area [38,48].Drying of several agricultural products, using the thin layer approach, in solar dryers and direct sunlight have been described in previous studies [38,40,42,47,49].However, the dehydration of banana stalk, using the thin layer approach, in a solar dryer or direct sunlight has not been reported.
Although the dehydration of agro-material is an energy-intensive and time-consuming operation, the drying rates of agro-products can be enhanced, and the drying time shortened by applying pretreatments prior to the drying operation [50].Drying pretreatments such as thermal (e.g.hot water blanching) and chemical (e.g.sulphiting and osmotic dehydration) treatments can soften material texture/tissue and change cell membrane permeability, thereby improving moisture migration in the material, increasing the rate of drying and shortening the time needed for drying [50][51][52][53].Hence, pretreatments have been applied to the drying of several agro-products especially vegetables and fruits [50,53].However, the effect of pretreatment on the solar and direct sunlight drying of the banana stalk has not been investigated yet.
Mass and heat transfer are coupled in a drying process, hence heat and mass transport properties of banana stalk chips e.g. the diffusivity of moisture are essential for dryer design.Thin layer drying mathematical models may be employed to evaluate the time required to dehydrate a material; they are suitable for the design, control as well as optimization of dehydration operations.Hence, these models are applied to depict the characteristics of agromaterials, including fruits and vegetables, during drying [47,49,54].This study sought to find out the effect of chip thickness and pretreatment on the solar and direct sunlight drying behaviour of banana stalk chip biomass.The study objectives included the estimation of the moisture diffusivities for the dehydration processes and description of the drying behavior by a fitting thin layer drying mathematical model.

Sample collection and preparation
Fresh banana stalks were obtained locally in Ogbomoso (8°8′N 4°14′E), Nigeria.The stalks which were of the equivalent diameter of about 33 mm were cut into 5-, 10-and 15-mm thick chips.Figs 1a, b are images of the banana stalk and chips, respectively.Some of the 5 mm thick banana stalk chips were subjected to hot water, salt water and sulphite pretreatments prior to the direct sunlight and solar dehydration operations to determine the influence of biomass pretreatment on the stalk dehydration behaviour.

Hot water pretreatment
This is a physical heat pretreatment process which is employed to decrease the microbial load of material, inactivate enzymes responsible for material deterioration, expel intracellular air to prevent material oxidation, remove wax from material surface, create fine cracks on skin of material, soften material tissue and change the cell permeability of the material [55][56][57].This thermal blanching process involves immersing the material in hot water at temperatures between 70 and 100 °C for several minutes [58].The removal of wax from material surface and creation of fine cracks on it as well as the softening of material tissue and changing of the cell permeability enhance the migration of moisture through the material and consequently its drying rate [55][56][57].
In this study, water was boiled at 100 °C and a set of 5 mm thick banana stalk chips was then soaked in the hot water for 10 min.The chips were afterwards taken out of the hot water, completely drained and placed in drying pans.

Saltwater pretreatment
This involves placing the material in hypertonic (salt) solution to create an osmotic pressure difference that drives the transport of water from the material to the salt solution, thus reducing the moisture present in the material and subsequently enhancing the overall rate of moisture removal from the material [50].
A salt solution was prepared by dissolving 300 g of common salt (NaCl) in 1 L of water.A set of 5 mm thick banana stalk chips was soaked in the salt solution for 10 min.The salt water treated chips were removed from the salt solution after 10 min, properly drained, and then placed in drying pans.

Sulphite pretreatment
Sulphite pretreatment is a chemical liquid phase pretreatment process which enhances the drying kinetics of agricultural produce by altering the permeability of the material cell membrane [50].Sulphite salts that are soluble in water such as potassium metabisulphite, sodium metabisulphite and sodium hydrogen sulfite or sulfur dioxide gas can be utilized for sulphite pretreatment [50].This pretreatment also prevents material deterioration and reduces the darkening of the dried material [59].
Potassium metabisulphite (K 2 S 2 O 5 ) solution was prepared by dissolving 2.5 g of the powder in 1 L of water at room temperature (28 °C).Another set of 5 mm thick banana stalk chips was then soaked in the solution for 10 min.The sulphite treated chips were then separated from the solution, totally drained and spread in drying pans.

Drying apparatus and procedure
Banana stalk chips of 5-, 10-and 15-mm thickness were placed in separate trays and then positioned in direct sunlight.A different set of drying trays containing chips of the same thickness 5, 10 and 15 mm was simultaneously placed in a locally fabricated solar dryer.The solar dryer that was utilized in this study, shown in Fig. 2, was a natural convection direct solar dryer.Fresh air flows, by natural convection, into the solar dryer chamber via openings at the lower sides of the dryer which are covered with wire mesh while the wet air flows out of the dryer through the chimney at its top as shown in Fig. 2. The biomass in the dryer was subjected to the radiation from the sun through the glass cover which is transparent.The initial masses of the 5, 10 and 15 mm thick banana stalk chips were 7.19, 12.49 and 17.2 g, respectively.In both cases of solar and open sun drying, the banana stalk chips were weighed, using a Mettler BB3000 weighing balance (accuracy of ± 0.1 g), prior to the commencement of drying operation and thereafter at 30 min interval till an unchanging mass was observed.
To establish the effect of pretreatments (hot water, salt water and sulphite) on the direct sunlight and solar drying characteristics, separate sets of dehydration pans containing each of the pretreated stalk chips were also placed in direct sunlight and the solar dryer simultaneously.The 5 mm thick chips were used for the investigation of the influence of Fig. 2 The natural convection (passive) direct solar dryer used in this study a schematic b dimensions pretreatment on the dehydration characteristics; the initial mass of each of the banana stalk samples was about 7.19 g.Each experiment was performed in triplicates.The drying operations took place between 9 am and 6 pm in June, in Ogbomoso, Nigeria.The ambient and solar dryer temperatures as well as the air humidity and velocity were measured during the dehydration operations.The temperature, relative humidity and velocity were measured using a PCE Instruments Kestrel 4000 NV weather tracker, which has accuracies of ± 1 °C, ± 3.0% RH and 3% of the air velocity reading, respectively.

Analysis
At any timet , the moisture present in the stalk, X t (g water g dry matter −1 ), is given as: where m t is mass (g) of stalk at time, t and m d is mass (g) of the completely dry stalk.The dimensionless moisture ratio ( M R ) is expressed as: where X i is initial moisture content and X e is equilibrium moisture content.In the case that the drying time is very long, X i and X t are very large compared toX e , so Eq. ( 2) becomes [60]: The rate of drying of the stalk is: where D R is the rate of drying of banana stalk measured in "g water/g dry matter.min", while X t+dt is moisture content at t + dt and dt is the increment in time (min).
The second law of diffusion given by Fick may be applied to depict moisture diffusion through the banana stalk when drying occurs during the falling-rate phase [38,61,62].The mechanism that controls the process during this period is internal mass transfer.With respect toM R , this law is expressed as [63]: where x is spatial dimension (m) and D eff is the effective moisture diffusivity (m 2 s −1 ).
The geometry of the banana stalk chip was considered as a slab, so if the assumptions of moisture migration in a slab (which was considered as infinite) in only one direction, constant diffusivity, negligible external resistant, even initial moisture distribution as well as insignificant shrinkage are made, the solution of Eq. ( 5) is [64]: For amply long dehydration time, the first term in the series expansion of Eq. ( 6) gives a suitable approximation of the solution [65]: where t is the dehydration time (min) and L is the slab thickness (m) when dehydration occurs from only one side.Equa- tion (7) may be expressed as: A straight line is obtained when In M R is plotted versus time t and D eff is calculated from the slope ( S ) of this line: The technique of nonlinear regression analysis was utilized to fit the moisture ratio-time data to the twelve models described in Table 1, using the Microsoft Excel software.It has been reported that these models most frequently best fitted the dehydration data of agricultural produce [49].The coefficient of determination (R 2 ), Chi-square ( 2), root mean square error (RMSE) and sum of square error (SSE) were the criteria employed to find the model which most suitably describe the drying operation.The model which most aptly fitted the dehydration data was the one that had the largest value of R 2 and least 2 , RMSE and SSE [49].The R 2 were computed by the RSQ function of Excel while 2 , SSE and RMSE were determined from Eqs. 10-12 by spreadsheet calculation in Excel.
where,z M R exp,i , N and M R pred,i are number of constants, experimental moisture ratio, number of observations and predicted moisture ratio, respectively.( 8) Henderson and Pabis 3 Results and discussion

Drying characteristics of banana stalk in open sun
Figure 3a, b show the temperature variation against the time of the day for the first drying day and second dehydration day, respectively.Similarly, Fig. 3c shows the plot of ambient and solar dryer temperatures against dehydration time which indicates the variation of the ambient and solar dryer temperatures during the dehydration process, with the drying times for both days combined.On the first day, the ambient temperature varied from 28.4 to 55.7 °C whereas on the second day ranged from 29 to 54 °C.In the case of the solar dryer, its temperature ranged from 38 to 77 °C on the first day and 32 to 72 °C on the second day.The solar dryer temperatures were higher than the ambient temperatures suggesting that higher thermal energy for vaporization of moisture from the banana stalk biomass was available in the solar dryer compared to direct sunlight.Figure 3d shows the plot of relative humidity against dehydration time for the two-day dehydration period.The relative humidity ranged between 56.5 and 72.5% on the first day and 56.3 and 75.6% on the second day.The measured velocity of natural air during the experiments ranged from 0.3 to 4.9 m s −1 .
Figure 4a depicts the graph of dimensionless moisture ratio against dehydration period for the dehydration of banana stalk chips of 5-15 mm thickness in direct sunlight.The moisture ratio diminished with time as moisture was gradually removed from the chips.The rate of dehydration increased whereas the time required for dehydration consequently decreased with decreasing thickness of the banana stalk chips; the time needed for chips dehydration decreased from 1350 min for the 15 mm thick chip to 780 min for the 5 mm thick chip.The drying time reduced with decreasing thickness of the banana stalk chip because the path required for diffusion of moisture through the stalk chip decreased with decreasing thickness of the chip, so moisture was removed faster through thinner chips [38,48,79,80].
Also, the initial mass of the banana stalk chip and the moisture present in it actually increased with increasing chip thickness, for a banana stalk of uniform diameter, so longer drying time was required to take out moisture from thicker chips.Hence, the rate of dehydration of the stalk chip can be significantly improved and consequently the time required for chips dehydration considerably shortened by reducing the chip thickness.Observable increase in the rate of drying of agricultural products as material thickness decreases has been previously reported in other studies [48,81].Figure 4b shows that the highest dehydration rates of 0.038, 0.025 and 0.018 g water/g dry matter.minwere observed during the dehydration of the 5-, 10-and 15-mm thick chips in open sun, respectively.The rate of dehydration largely diminished with time and was generally in the falling rate period.A falling-rate phase indicates that the rate of the open sun drying of banana stalk chip was limited by the migration of moisture from the inside of the chip to its surface [61,62].Akpinar [82], Akpinar and Bicer [54] and Akpinar [83] have similarly observed a falling rate phase during the drying of some agroproducts in direct sunlight.Diffusivities of moisture in the 5-, 10-and 15-mm banana stalk chips during their drying in direct sunlight were 1.22 × 10 -9 , 2.16 × 10 -9 and 3.65 × 10 -9 m 2 s −1 , respectively.The observed rise in diffusivity with increasing chip thickness is deemed to be due to the higher moisture activity associated with larger initial moisture present in thicker chip samples.A rise in diffusivity with increasing depth or thickness of material has similarly been stated in the literature [79,84,85].The observed diffusivities, in this investigation, for the drying of banana stalk biomass in direct sunlight are within 10 -12 -10 -6 m 2 s −1 earlier observed during the dehydration of other agro-materials [47].
The graph of moisture ratio against time required for direct sunlight dehydration of 5 mm thick untreated, hot water treated, salt water treated and sulphite treated banana stalk chips is shown in Fig. 5a.As expected, moisture ratio diminished gradually as drying advanced showing that moisture was steadily taken out of all the chips samples by the sun radiation.The moisture ratios of the hot water treated chips were significantly (P < 0.05) lesser than those of the untreated stalk during the dehydration process until the moisture present in the stalk reached equilibrium.However, the moisture ratio of the salt water and sulphite treated stalk were only significantly (P < 0.05) lesser than those of the untreated samples for the first 360 and 330 min of the dehydration process, correspondingly.Similar to the untreated chip, the rate of drying of the pretreated chips decreased with increasing time as shown in Fig. 5b, indicating that the dehydration process occurred entirely in the falling-rate phase and the open sun dehydration of hot water, salt water and sulphite pretreated banana stalk chips was also limited by the migration of moisture from the inside of the chip to its exterior [61,62].The initial drying rates of the hot water, salt water and sulphite pretreated banana stalk chips were 0.084, 0.028 and 0.045 g water/g dry matter.min,correspondingly.Diffusivities of moisture in the hot water treated, salt water treated and sulphite treated banana stalk chips during their drying in direct sunlight were 1.44 × 10 -9 , 1.11 × 10 -9 and 1.08 × 10 -9 m 2 s −1 , correspondingly.The effective moisture diffusivity of the hot water treated chips (1.44 × 10 -9 m 2 s −1 ) was significantly (P < 0.05) larger than that of the untreated chips (1.22 × 10 -9 m 2 s −1 ), however, those of the salt and sulphite treated chips were significantly (P < 0.05) lesser than that of the untreated stalk.

Solar drying characteristics
Figure 6a shows that the 5, 10 and 15 mm thick banana stalk chips were dehydrated in the solar dryer in 570, 780 and 990 min, respectively; this observed rise in drying time with increasing chip thickness is also as a result of the larger initial mass and moisture present in the stalk, in addition to the longer path required for moisture diffusion by the thicker chips compared to those required by the thinner chips [38,48,79,80].This result also suggests that the rate of the solar drying of banana stalk chips can be significantly increased, and the drying time considerably reduced by using very thin layers of the chips.
The drying times of 570-990 min required for the dehydration of the 5-15 mm banana stalk chips in the solar dryer were significantly lower than those of 780-1350 min similarly required for the dehydration of the 5-15 mm thick chips in direct sunlight.This is attributable to the elevated temperatures attained in the dryer compared to external temperatures as shown in Fig. 3a-c.The ambient temperature ranged from 28.4 to 55.7 °C on the first day and 29 to 54 °C on the second day; however, the solar dryer temperature varied from 38 to 77 °C and 32 to 72 °C on the first day of drying and second day of dehydration, respectively.The higher temperatures of the solar dryer imply the availability of larger heat energy for drying in the solar dryer relative to the open sun.The larger thermal energy provided higher drying rates in the solar dryer which resulted in the observed shorter drying times.Consequently, solar energy may be more effectively utilized for dehydrating the banana stalk chips biomass by using solar dryers instead of drying directly in sunlight or open sun.
Figure 6b depicts the graph of dehydration rate against the time required for dehydrating banana stalk chips of 5-, 10and 15-mm thickness in a solar dryer.The drying rate initially increased but then generally declined as drying advanced; dehydration rates were chiefly in the falling-rate phase.Hence, solar dehydration of banana stalk chip was essentially limited by migration of moisture from the internal portion of the chip to its exterior.The maximum drying rates of the 5-, 10-and 15-mm thick chips were 0.040, 0.026 and 0.022 g water/g dry matter.min,correspondingly.Diffusivities for drying the 5-, 10-and 15-mm thick chips in the solar dryer were 1.28 × 10 -9 , 3.17 × 10 -9 and 5.32 × 10 -9 m 2 s −1 , respectively.They are significantly larger than those of 1.22 × 10 -9 , 2.16 × 10 -9 and 3.65 × 10 -9 m 2 s −1 for the dehydration of chips of 5-, 10-and 15-mm thickness, respectively, in direct sunlight.The higher diffusivities of moisture in the material during drying in the solar dryer were due to the rise in the activity of water molecules in banana stalk chips as a result of the rise in the thermal energy available in the solar dryer relative to drying in direct sunlight [86].Figure 7a depicts the graph of moisture ratio against time for the drying of untreated, hot water treated, salt water treated and sulphite treated 5 mm thick banana stalk chips in the solar dryer.The moisture ratio of the hot water treated, salt water treated and sulphite treated stalk chips were significantly (P < 0.05) lesser than those of untreated stalk.Also, lower drying times were required for the treated chips relative to the untreated chips.The time required for the solar drying of the hot water, salt water and sulphite treated chips were 510, 540 and 450 min, respectively, compared to the drying time of 570 min required for the untreated chips.The hot water pretreatment softened the banana stalk tissue and changed cell membrane permeability, thus enhancing moisture diffusion through the material, increasing the rate of biomass drying and shortening the time needed for dehydrating the stalk [50,52,56,57].The osmotic pressure created by dipping the stalk chips in saltwater caused the transport of moisture from the chips into the hypertonic solution which consequently led to the observed reduction in drying time of the saltwater treated banana stalk chips [50].The sulphite pretreatment favourably altered the permeability of the material cell membrane thus improving the drying rates of the banana stalk chips [50].Kumar et al. [87] have also reported a shortening of solar drying time of mushrooms due to pretreatment.Figure 7b shows that the drying rates of both untreated and pretreated chips declined with increasing drying time.The rates are generally in the falling-rate phase, so the solar dehydration of both treated and untreated chips was limited by the migration of moisture from the inside of the chip to its exterior.The highest drying rates measured for the solar drying of the hot water and sulphite pretreated banana stalk chips of 0.072 and 0.070 g water/g dry matter.min,respectively, were significantly larger than that of 0.040 g water/g dry matter.minfor the solar dehydration of the untreated chip.The effective moisture diffusivities of 2.11 × 10 -9 , 1.99 × 10 -9 and 1.65 × 10 -9 m 2 s −1 measured for the solar drying of hot water, salt water and sulphite treated 5 mm thick banana stalk chips, respectively, were significantly (P < 0.05) higher than 1.28 × 10 -9 m 2 s −1 measured for the untreated stalk chips.This observation confirms that the pretreatments significantly enhanced the diffusion of moisture through the banana stalk chips and consequently decreased the drying time.

Thin layer modeling of drying behaviour
The statistical parameters and model constants obtained when the twelve models described in Table 1 were fitted to the dehydration data for the solar and open sun drying of untreated, hot water treated, salt water treated, and sulphite treated banana stalk chips are presented in Tables 2, 3, 4 and 5. Table 2 indicates that the Midilli-Kucuk model possessed the highest values of R 2 (≥ 0.997) and the least SSE (≤ 0.00021), RMSE (≤ 0.0144) and χ 2 (≤ 0.00023) values when the data obtained for direct sunlight dehydration of 5, 10 and 15 mm thick untreated banana stalk chips were fitted to the twelve models.However, Table 3 shows that the Weibull model most suitably fitted the data for the drying of hot water treated, salt water treated, and sulphite treated banana stalk chips in direct sunlight; this model had the highest values of R 2 (≥ 0.990) and the least SSE (≤ 0.00077), RMSE (≤ 0.0277) and χ 2 (≤ 0.00090) values.Table 4 indicates that the Weibull model yielded the highest R 2 (≥ 0.998) and the least SSE (≤ 0.00018), RMSE (≤ 0.0133) and χ 2 (≤ 0.00022) when the twelve models were fitted to the data for the solar drying of 5, 10 and 15 mm thick untreated banana stalk chips.Similarly, as indicated in Table 5, the same Weibull model most aptly described the solar drying of hot water treated, salt water treated and sulphite treated banana stalk chips; this model possessed the largest values of R 2 (≥ 0.996) and the least SSE (≤ 0.00030), RMSE (≤ 0.0173) and χ 2 (≤ 0.00038) values when it was fitted to the solar drying data.
Figure 8 shows graphs of predicted moisture ratios against experimental moisture ratios; it is evident that the Weibull model well explained the solar dehydration while the Midilli-Kucuk and Weibull models aptly explained the open sun dehydration of untreated and pretreated banana stalk chips, respectively.Other studies have also reported that the Midilli-Kucuk model most suitably explained the open sun dehydration of long green pepper [54] and grapes [88] while the Weibull model most aptly explained the direct solar drying of banana peel [89] and Stevia leaves [90].

Conclusions
The effects of chip thickness and drying pretreatments on the solar and open sun dehydration of banana stalk chips were investigated.The dehydration of both solar and direct sunlight drying of the stalk chips occurred mainly in the falling-rate phase.The time required for both dehydration operations were shortened by decreasing the chip thickness as was expected.However, the stalk chips dried faster in the solar dryer than in direct sunlight because higher thermal energy (as depicted by higher temperatures) was available for biomass dehydration in the solar dryer compared to drying at ambient temperatures.Hot water pretreatment significantly (P < 0.05) increased the rate of removal of moisture from the banana stalk chips during the dehydration in direct sunlight until the moisture present in the chips reached equilibrium.More interestingly, the hot water, salt water and sulphite pretreatments of the banana stalk chips significantly (P < 0.05) increased the moisture diffusivity and shortened the time required for the solar drying of the stalk chips.The effective moisture diffusivities of 1.08 × 10 -9 -3.65 × 10 -9 m 2 s −1 and 1.28 × 10 -9 -5.32 × 10 -9 m 2 s −1 , calculated for the open sun and solar dehydration of banana stalk chips, respectively, were within 10 -12 -10 -6 m 2 s −1 earlier evaluated for the dehydration of agro-materials.The Weibull model most aptly explained the solar dehydration of both untreated (5-15 mm thick) and pretreated (hot water treated, salt water treated and sulphite treated) banana stalk chips.On the   The abundant, free, renewable and environmentally benign solar energy may be more effectively utilized for drying banana stalk chips biomass by using very thin stalk chips, pretreating the chips before drying (e.g. by hot water, salt water or sulphite) and by drying them in a solar dryer in preference to direct sunlight.The use of solar energy for dehydrating banana stalk biomass would reduce its processing cost and enhance the sustainability of its utilization.Funding The authors did not receive support from any organization for the submitted work.

Fig. 1
Fig. 1 Images of banana waste biomass a stalk b stalk chips

Fig. 3 a
Fig. 3 a Graph of solar dryer and ambient temperatures against time of the day for the first day of dehydration operation b Graph of solar dryer and ambient temperatures against time of the day for the second day of dehydration operation c Graph of solar dryer and ambient temperatures against dehydration time for a two-day dehydration period d Graph of relative humidity against dehydration time for a twoday dehydration period

Fig. 4 a
Fig. 4 a Graph of moisture ratio against time for open sun drying of 5, 10 and 15 mm thick banana stalk chips b Graph of drying rate against time for open sun drying of 5, 10 and 15 mm thick banana stalk chips

Fig. 5 a
Fig. 5 a Graph of moisture ratio against dehydration time for the dehydration of untreated and pretreated 5 mm thick banana stalk chips in direct sunlight b Graph of rate of drying against time required for drying untreated and pretreated 5 mm thick banana stalk chips in direct sunlight

Fig. 6 aFig. 7 a
Fig. 6 a Graph of moisture ratio against dehydration time for the solar dehydration of 5-, 10-and 15-mm thick banana stalk chips b Graph of rate of drying against time needed for the solar drying of 5-, 10-and 15-mm banana stalk chips a = 1.541, c = − 0.489, k = 0.002 other hand the Midilli-Kucuk model most aptly explained the open sun dehydration of untreated 5-15 mm thick banana stalk chips while the Weibull model most suitably explained the open sun drying of hot water treated, salt water treated and sulphite treated banana stalk chips.

Table 2
Statistical parameters and constants obtained after models were fitted to the data for open sun dehydration of untreated 5-, 10and 15-mm thick banana stalk chips

Table 3
Statistical parameters and constants obtained after models were fitted to the data for open sun dehydration of 5 mm thick hot water treated, salt water treated, and sulphite treated banana stalk chips

Table 4
Statistical parameters and constants obtained after models were fitted to the data for solar dehydration of untreated 5-, 10-and 15-mm thick banana stalk chips

Table 5
Statistical parameters and constants obtained after models were fitted to the data for solar dehydration of 5 mm thick hot water treated, salt water treated, and sulphite treated banana stalk chips